The present invention relates to switching power supplies. In particular, the invention relates to a method and apparatus for permitting a switching power supply to produce a regulated output voltage at lower input voltages than was possible in the prior art.
Switching power supplies (SPS) are known in the art. Typical uses of switching power supplies include generating a constant, regulated output voltage Vout from an input source, whose voltage Vin may be decreasing in amplitude over time. As an example, a switching power supply may be used to generate a regulated output voltage Vout from the potential difference across terminals of a battery in order to operate an electrical device such as a portable computer, a radio, a CD player, or the like. As the electrical device drains energy from the battery, the input voltage Vin supplied by the battery diminishes over time, and it is the job of the switching power supply to continue generating a steady output voltage Vout to keep the electrical device operating.
In designing switching power supplies, the challenge has been to keep a switching power supply in regulation for as long as possible, i.e., to continue generating the regulated output voltage Vout for as long as possible as the input voltage Vin drops toward zero. To facilitate discussion, FIG. 1 is an illustration of an input voltage Vin line versus a regulated output voltage Vout line for a typical prior art switching power supply (SPS).
In the example of FIG. 1, when Vin is high, e.g., 7 V, the SPS produces a regulated output voltage Vout at say 5 V.
When Vin drops below a threshold, say 5.5 V, the SPS no longer stays in regulation, and the output voltage Vout can no longer be maintained at its regulated 5 V value. FIG. 1 is representative of the Vin/Vout characteristics of a typical SPS since typical SPS's usually go out of regulation when their input voltage Vin come within 10% of their regulated output voltage Vout. On the Vout line, the point where the SPS goes out of regulation is shown as point 200. The difference between the input voltage Vin and the regulated output voltage Vout at the point where the SPS goes out of regulation is called the drop-out voltage. In the example of FIG. 1, the drop-out voltage is 0.5 V (5.5 V-5 V).
It has been found that by reducing the drop-out voltage, i.e., the difference between the input voltage Vin and the output voltage Vout at the point where the SPS goes out of regulation, it is possible to generate the desired regulated output voltage Vout at lower input voltages. The ability to continue generating a regulated output voltage Vout for a longer period of time responsive to a diminishing input voltage Vin is highly desirable because the longer the SPS can stay in regulation, the longer the electrical device which depends on the SPS and the input source for its energy needs can operate. By way of example, SPS's with low drop out voltages may permit portable computers to operate for a longer period of time on their batteries.
Consequently, what is desired is an improved method and apparatus for permitting a switching power supply (SPS) to stay in regulation at lower input voltages than was possible in the prior art.